1. Field of the Invention
The present invention relates generally to the art of transferring a thin plate material such as paper or the like (simply called "sheet" hereinbelow), and more particularly to a sheet transfer device for transferring a sheet discharged from a nipping region between a printing drum and a back press roller of a rotary printer with one surface thereof being applied with a printing according to a rotation of the printing drum to a sheet receiving means such as a sheet receptacle.
2. Description of the Prior Art
The Belt conveyers are well known as a means for transferring articles, and of course a transfer of sheet can also be readily accomplished by using a belt conveyer. In the art of printing machines, a sheet transfer device using a belt conveyer for transferring a sheet discharged from a nipping region between a printing drum and a back press roller with a surface thereof being applied with a printing according to a rotation of the printing drum to a sheet receptacle has been proposed by Japanese Patent Laid-open Publication 60-148864, wherein a rear surface portion of the belt conveyer is evacuated to hold the sheet on the belt conveyer by vacuum. By the rear surface of the belt conveyer being evacuated, a sheet applied with a printing on its upper surface is definitely held on the belt conveyer, requiring no means to directly touch the upper surface of the printed sheet for the sheet being definitely transferred to the sheet receptacle, together with an additional function available that the sheet which would remain as adhered onto the printing drum due to a strong viscosity of printing ink as in a stencil printing is effectively peeled off from the printing drum with no means to touch the printed surface of the sheet. Further, as was proposed in the above-mentioned publication, when raised portions are provided at an end portion of the sheet transfer route of the belt conveyer so as to engage the rear surface of the sheet at opposite side edge portions thereof so as thereby to lift the opposite side edge portions of the sheet relative to a central portion thereof held on the conveyer by vacuum, so that the sheet is thereby bent into a U-shape according to the progress of transfer, even a thin sheet is endowed with a rigidity which contributes to transferring the sheet definitely to the very end of the sheet receptacle.
However, in the construction that a sheet is transferred by a belt conveyer evacuated at the rear surface thereof as held thereon by vacuum as described above, it is required that the operation speed of the belt conveyer is correctly synchronized with the rotation speed of the priming drum, and therefore a large scaled drive connection mechanism is required for drivingly connecting the belt conveyer with the priming drum, thereby inducing a high cost of the printing machine, and in addition the increase of the printing speed is obstructed by the inertia of the drive connection mechanism. Further, in order to evacuate the rear surface of the belt conveyer, a relatively large housing is required to enclose the rear side of the belt conveyer, and a fan of a large capacity is required to evacuate a large amount of air from the inside of such a large housing, obstructing down sizing of the printing machine and reduction of the manufacturing cost.